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Open Access Publications from the University of California

UC Santa Barbara Previously Published Works

Cover page of Britannicus or The Secrets of Space

Britannicus or The Secrets of Space

(2107)

The secret motives and decisions of the play are generally confined to the wings where no prying eye or ear, including the spectator's, can perceive them. The stage then becomes an intermediary present between the past and another present happening out of sight but just as significant.

Cover page of ChemFate: A fate and transport modeling framework for evaluating radically different chemicals under comparable conditions.

ChemFate: A fate and transport modeling framework for evaluating radically different chemicals under comparable conditions.

(2020)

With the ever-increasing development on novel chemicals and materials, with radically different properties and environmental behavior, it is challenging to compare their environmental behavior under similar conditions. For example, pesticides may be non-ionizable organics, ionizable organics, metal ion-based, or nanomaterials. These very different chemicals behave very differently. To date, no single modeling platform can handle all adequately, in a dynamic framework that accounts for actual variability in meteorology, rates of release to the environment, and the actual characteristics of the region of interest. Here we present ChemFate, a framework to address this challenge, by incorporating four different fate and transport models, each to address these four classes of chemicals (non-ionizable organics, ionizable organics, metal ion-based, or nanomaterials). We build upon established models, but have incorporated a number of additional processes. After demonstrating that the individual models comparable favorably with observed data and the previous models, under similar conditions, we conducted a case study with four radically different fungicides, used in the Central Valley, California. We found that although the concentrations of the non-ionizable and ionizable organic fungicide spike after application and temporarily may exceed toxicity thresholds for Daphnia Magna, they do not accumulate over time, while the metal ion and metallic nanoparticle result in increasing accumulation of Cu2+, eventually exceeding the toxicity threshold during runoff events. This case study demonstrates the value of a framework that allows the comparison of different classes of chemicals under the same conditions.

Cover page of Entrainment of particles during the withdrawal of a fiber from a dilute suspension

Entrainment of particles during the withdrawal of a fiber from a dilute suspension

(2020)

A fiber withdrawn from a bath of a dilute particulate suspension exhibits different coating regimes depending on the physical properties of the fluid, the withdrawal speed, the particle sizes, and the radius of the fiber. Our experiments indicate that only the liquid without particles is entrained for thin coating films. Beyond a threshold capillary number, the fiber is coated by a liquid film with entrained particles. We systematically characterize the role of the capillary number, the particle size, and the fiber radius on the threshold speed for particle entrainment. We discuss the boundary between these two regimes and show that the thickness of the liquid film at the stagnation point controls the entrainment process. The radius of the fiber provides a new degree of control in capillary filtering, allowing greater control over the size of the particles entrained in the film.

Cover page of Air entrainment and granular bubbles produced by an impinging jet of grains into water

Air entrainment and granular bubbles produced by an impinging jet of grains into water

(2020)

A jet of water entering into a pool of the same liquid can generate air entrainment and form bubbles that rapidly raise to the surface and disintegrate. Here we report the equivalent phenomenon produced by a plunging dry granular jet, so far unexplored. For grains smaller than a critical size, the granular jet entering into the pool produces air entrainment that leads to bubbles formation. The bubbles formed are covered by a shell of grains attached to the bubble air-water interface due to capillary-induced cohesion. In contrast to classical air bubbles, these "granular bubbles" are stable over time because the granular shell prevents coalescence and keeps the air encapsulated either if the bubbles rise to the surface or sink to the bottom of the pool, which is determined by the competition of the buoyant force and the weight of the assembly. Experimentally, we show how the bubble size and volume of entrained air depend on the grain size, liquid properties and jet impact velocity.

Cover page of Dynamic Contagion in a Banking System with Births and Defaults

Dynamic Contagion in a Banking System with Births and Defaults

(2020)

We consider a dynamic model of interconnected banks. New banks can emerge, and existing banks can default, creating a birth-and-death setup. Microscopically, banks evolve as independent geometric Brownian motions. Systemic effects are captured through default contagion: as one bank defaults, reserves of other banks are reduced by a random proportion. After examining the long-term stability of this system, we investigate mean-field limits as the number of banks tends to infinity. Our main results concern the measure-valued scaling limit which is governed by a McKean-Vlasov jump-diffusion. The default impact creates a mean-field drift, while the births and defaults introduce jump terms tied to the current distribution of the process. Individual dynamics in the limit is described by the propagation of chaos phenomenon. In certain cases, we explicitly characterize the limiting average reserves.

Cover page of Transcending the ensemble: baby universes, spacetime wormholes, and the order and disorder of black hole information

Transcending the ensemble: baby universes, spacetime wormholes, and the order and disorder of black hole information

(2020)

In the 1980's, work by Coleman and by Giddings and Strominger linked the physics of spacetime wormholes to `baby universes' and an ensemble of theories. We revisit such ideas, using features associated with a negative cosmological constant and asymptotically AdS boundaries to strengthen the results, introduce a change in perspective, and connect with recent replica wormhole discussions of the Page curve. A key new feature is an emphasis on the role of null states. We explore this structure in detail in simple topological models of the bulk that allow us to compute the full spectrum of associated boundary theories. The dimension of the asymptotically AdS Hilbert space turns out to become a random variable $Z$, whose value can be less than the naive number $k$ of independent states in the theory. For $k>Z$, consistency arises from an exact degeneracy in the inner product defined by the gravitational path integral, so that many a priori independent states differ only by a null state. We argue that a similar property must hold in any consistent gravitational path integral. We also comment on other aspects of extrapolations to more complicated models, and on possible implications for the black hole information problem in the individual members of the above ensemble.